Electrothermal heater made from thermally conducting electrically insulating polymer material

ABSTRACT

Thermally conductive films, composite materials including the films, and electrothermal heaters including the films, are disclosed. The films include a polymer and a sufficient concentration of hexagonal boron nitride to provide adequate heat transfer properties, and have high thermal conductivity, peel strength, and shear strength. The films can include thermoset polymers, thermoplastic polymers, or blends thereof, and can also include electrically conductive materials, reinforcing materials such as fiberglass, carbon fiber, metal mesh, and the like, and thermally conductive fillers, such as aluminum oxide, aluminum nitride, and the like. The films can be included in composite materials. The films can be used as part of a layered structure, and used in virtually any application, for example, various locations in aircraft, where heating is desirable, including nacelle skins, airplane wings, heated floor panels, and the like. The electrothermal heaters provide a more even heat, and a more rapid heat, than current resistive heaters formed from metal foils adhered to an adhesive film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally in the field of polymeric films orcomposite materials including hexagonal boron nitride, compositematerials formed from the films, and electrothermal heaters and/or heatsinks formed from the films or composite materials.

2. Description of the Related Art

There are currently various electrothermal de-icing or anti-icingproducts for leading edge ice protection of aero surfaces. Thesetypically use heating elements or electrodes disposed at a leading edgesurface of the aero structure in the form of a serpentine orinterdigitated finger area grid to deliver heat to any ice formed on thesurface. Typical electrothermal de-icing or anti-icing products utilizean embedded heating element, which is located below the externalsurface. Heat generated by electrothermal element must pass through athick layer of thermally insulating polymer matrix composite prior totransfer of heat to the surface-ice interface.

It would be advantageous to provide new electrothermal ice protectionsystems which facilitate the rapid transfer of thermal energy to thesurface-ice interface, providing energy savings and enhancedfunctionality. It would further be advantageous to provideelectrothermal ice protection systems rugged enough to withstandexposure to an aero structure operational environment, and capable ofshedding ice from an aero surface at safe voltages and power levels,ideally before the ice accretes to produce any appreciable thickness.The present invention provides such systems.

SUMMARY OF THE INVENTION

Films comprising a polymer and a sufficient concentration of hexagonalboron nitride to provide adequate heat transfer properties for use inelectrothermal heating applications are disclosed. Composite materialsincluding these films, and electrothermal heaters formed from thesecomposite materials, are also disclosed.

The films comprise a polymer and hexagonal boron nitride. The polymercan be a thermoset polymer, a thermoplastic polymer, or a blend thereof.The film can also include other components, for example, electricallyconductive materials, including nanoparticulate materials such as carbonnanotubes, carbon nanofibers, metal nanowires, metal-coated glassmicrobubbles, graphene sheets, and the like. These materials aretypically present in a range of from about 10 percent to about 60percent by weight of the hexagonal boron nitride. Also, the films can bereinforced with various materials, such as fiberglass, carbon fiber,metal mesh, and the like, and thermally conductive fillers, such asaluminum oxide, aluminum nitride, and the like, can optionally be added.

The films can be formed by blending polymerizable monomers and hexagonalboron nitride, spreading the blend into a film thickness, and curing themonomers. In some embodiments, the blend is fairly viscous at roomtemperature, for example, the viscosity of paste or putty, and can bewarmed to temperatures above room temperature to thin the blend to makeit easier to spread. The films can also be formed by blendingthermoplastic polymers and hexagonal boron nitride, and casting theblend into a film. In one aspect, the thermoplastic polymers are meltedor softened before blending, or the blend is heated to an elevatedtemperature to facilitate spreading. The melted polymers can be cooledto form the resulting film.

The films ideally have suitable physical properties for inclusion inlaminates subjected to a wide range of environmental conditions. Theseproperties include high thermal conductivity, peel strength, and shearstrength. The thermal conductivity must be suitably high to permit rapidheating. The peel strength of the film must be suitably high to preventdelamination. The shear strength must also be relatively high for thefilm to have desirable properties.

In one aspect, the thermally conductive films are included in acomposite material that includes an insulating layer, an electricallyconductive film layer, and thermally conductive film layer whichincludes hexagonal boron nitride. As current is passed through theelectrically conductive film layer, and the layer heats up, the heatpasses through the electrically conductive layer but not, at least to asignificant extent, through the insulating layer. Thus, this compositematerial can be used in electrothermal heating applications.

In another aspect, the thermally conductive films include, in additionto the hexagonal boron nitride, sufficient electrically conductivematerials, such as electrically conductive nanoparticulate materials,such that the films can both produce heat and conduct the heat. When acomposite material including this type of film and a thermallyinsulating film is used, it can perform substantially the same functionincluding the thermally insulating layer, the electrically conductivelayer, and the thermally conductive layer, but with one less layer.

In a third aspect, the thermally conductive film includes, in additionto the hexagonal boron nitride, electrically conductive materials, in anamount sufficient to increase the thermal conductivity of the layer, butinsufficient to cause the layer to be electrically conductive.

Thus, depending on the desired use, the films can act as resistors, andnot conduct electricity, or they can conduct electricity, where thedegree of conductance of the film can be modulated by including, inaddition to the hexagonal boron nitride, various amounts of electricallyconductive nanoparticulate materials.

The films, and composite materials including the films, can be includedas part of the composite material used to form a wing, nacelle, or otherouter surface of an aeroplane and serve to de-ice the wing, nacelle, orother outer surface. Alternatively, the films and composites can be usedin other heating applications, for example, floor panels, water tanks,pipes, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a composite material comprising athermally conductive film of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In accordance with the present invention, thermally conductive films,composite materials formed from the films, composite materials includingthe films, and electrothermal heaters including the composite materials,are all described herein. The present invention will be betterunderstood with reference to the following detailed description.

In one embodiment, the composition that can be formed into a thermallyconductive film comprises a polymerizable monomer and hexagonal boronnitride. In another embodiment, the composition comprises a meltablethermoplastic resin and hexagonal boron nitride.

The boron nitride powder used to prepare the films described herein isnot limited by any particular type of crystalline system, shape and sizeof powder grain, cohesion degree of powder particle or particledistribution. With respect to the crystalline form, boron nitride powderof hexagonal, cubic, wurtzite, rhombohedral, or any other crystal formscan be used. Among them, hexagonal boron nitride powder of thecommercially available hexagonal form, which provides heat conductivityof roughly 10 to 100 W/mK or so, or of cubic structure presenting anextremely high heat conductivity of 1300 W/mK maximum, can be used.

The particle shape of hexagonal boron nitride powder is not limited toscaled, or flat shape, but hexagonal boron nitride powder of variousparticle forms such as granular, lump, spheric, fiber, whisker-shapedhexagonal boron nitride powder, or ground products of these can be used.The particle diameter of hexagonal boron nitride powder can vary,however, the individual average primary diameter in the range of 0.01 to100 μm, or more preferably, in the range of 0.1 to 20 μm can be used. Nopractical limit is found with regard to the minimum hexagonal boronnitride particle size, while a hexagonal boron nitride powder of largerthan 100 μm is difficult to produce and is not easily entrained intothin polymer films. When scaled hexagonal boron nitride powder is used,a range of 0.5 to 50 μm as maximum diameter can be easily blended intothe film, and may be oriented using a magnetic field, if desired.Additionally, hexagonal boron nitride powder in the form of coheredprimary particles can be used.

In some embodiments, the hexagonal boron nitride has a bi-modal particlesize, whereby the bimodal particle size allows for more intimate contactof particles, and enhanced thermal conductivities, even at lower overallparticle content.

The concentration of hexagonal boron nitride powder in the heatconductive film is typically in the range of from about 12 percent toabout 40 percent by weight, based on the total weight of the film. Inone embodiment, the film comprises nanoparticulate materials, in whichcase a range of from about 0.2 percent to about 2.0 percent by weight ofhexagonal boron nitride powder is preferred. If more than thesepreferred amounts are used, the viscosity of the composition increasesand the fluidity decreases, making the handling difficult. Also, airbubbles can enter making it difficult to form a film while the shearstrength and other physical properties can suffer.

The hexagonal boron nitride can be, but need not be, aligned. Thealignment of the particles can be performed, for example, using amagnetic field using means known to those of skill in the art. Also, thehexagonal boron nitride particles can be surface treated prior to useusing means known to those of skill in the art.

The films typically include thermoset monomers capable of formingthermoset polymers. Conventional thermoset resin systems which can beused to form the films include, for example, epoxy based resin systems,matrices of bismaleimide (BMI), phenolic, polyester, PMR-15 polyimide,acetylene terminated resins, acrylics, polyurethanes, free-radicallyinduced thermosetting resins, and the like. As a result of suchconsiderable choices in thermosetting resins, the primers, paints and/orfilms of the invention can be tailored as desired.

Suitable epoxy resins include those used in established thermosetepoxy/fiber reinforced prepregs used in manufacturing aircraftcomponents. They are frequently based, inter alia, on one or more ofdiglycidyl ethers of bisphenol A (2,2-bis(4-hydroxyphenyl)propane) orsym-5 tris(4-hydroxyphenyl)propane, tris(4-hydroxyphenyl)methane,bispheniol F, tetrabromobisphenol A, their polyepoxide condensationproducts, cycloaliphatic epoxides, epoxy-modified novolacs(phenol-formaldehyde resins) and the epoxides derived from the reactionof epichlorohydrin with analine, o-, m- or p-aminophenol, and methylenedianaline.

The epoxy resin systems contain epoxy curing agents which cure the resinto a solid, infusible product. For this purpose, epoxy curing agentswhich are acidic, neutral or alkaline may be used. Examples include,among others, amine hardeners, phenols, acid anhydrides, polyamides andLewis acids and bases. Accelerators may also be used to decrease thecure time and include imidazoles and substituted ureas.

The amount of the hardener employed is usually stoichiometricallyequivalent on the basis of one amine group per epoxy group in the resin,Some adjustment of the stoichiometry may be required with the additionof the nanoreinforcement.

The hexagonal boron nitride and, optionally, other components, can beadded to thermoset monomer, hardener, or mixed resin. The method ofdispersion will depend on when the hexagonal boron nitride is added. Forexample, if the hexagonal, boron nitride is added to a B-staged resin,the high viscosity may require heating and ultrasonic dispersion or highshear mixing. The hexagonal boron nitride can also be deposited onto thefilm surface using heat or adhesive to hold it in place during compositeprocessing.

The hexagonal boron nitride can also be added to a thermoplastic polymerformed from thermoset monomers for consolidation with a thermoplasticstructure, or bonding with a thermoset structure. In a preferredembodiment, the hexagonal boron nitride is present in concentrations offrom about 5 percent to about 50 percent by weight based on the weightof the thermoplastic polymer. Conventional thermoplastic systems whichcan be used include, for example, polyetheretherketone (PEEK),polyetherketone (PEK), polyphenylenel sulfide (PPS), polyethylenesulfide (PES), polyetherimide (PEI), polyvinylidene fluoride (PVDF),polysulfone (PS), polycarbonate (PC), polyphenylene ether/oxide, nylons,aromatic thennoplastic polyesters, aromatic polysulfones, thennoplasticpolyimides, liquid crystal polymers, thermoplastic elastomers, and thelike.

The hexagonal boron nitride can be added to the thermoplastic, which canthen be mixed with a thermoset, before cure or vice versa. The hexagonalboron nitride can be added to one thermoplastic, which is then mixedwith another thermoplastic (e.g., pellets made and then extrudedsimultaneously).

In addition to the polymer and hexagonal boron nitride, the compositionused to form the film can include additional components, for example,those which enhance the strength of the film and those which enhance theelectrical conductivity of the film.

Examples of materials that enhance the strength of the film includematerials commonly found in pre-pregs, such as carbon fibers,fiberglass, metal wires or mesh, and the like. Examples of materialsused to enhance the conductivity of the film include nanoparticulatematerials, which are described in more detail below.

Electrically conductive materials added to the hexagonal boron nitridecontaining film may allow the film to double as both the heat source andas a means for delivering heat quickly to the ice interface. In thisconfiguration it is desirable to provide rapid heat transfer to the iceinterface by placing an electrically insulating, thermally conductinglayer between the heat source and the external skin, while placing anelectrically and thermally insulating layer on the backside to preventheat losses away from the ice interface. In total, this configurationdirects thermal energy to the ice interface.

An embodiment of this configuration would be the use of hexagonal boronnitride particles in combination with a carbon fiber or woven carbonfabric containing pre-preg. The combination of hexagonal boron nitrideand carbon fabric will allow for rapid and efficient transfer of energyaway from the carbon fabric heat source. The ability to rapidly transferheat and equally rapidly stop the transfer of heat to the ice interfaceprovides the simultaneous benefits of efficient ice shedding andlimiting of runback and refreezing of shed ice.

In one aspect, the films include powders, such as copper powder, carbonnanotubes or nanofibers which are also known as a type of multi-walledcarbon nanotubes (collectively, carbon nanotubes), and which aredistributed throughout the carrier, ideally in a substantiallyhomogenous fashion.

In this aspect, the powders typically have a particle size in the rangeof 4 nm to 100 μm. The particles can be irregular in shape or, smoothand round, or have texture. One example of a suitable textured particleis a “spiky” copper powder where the carbon nanotubes are embedded intothe copper. While not wishing to be bound by a particular theory, it isbelieved that the presence of the carbon nanotubes brings the CTE of thepolymer closer to that of the metal powder, such that the material canconduct sufficient electrical energy to provide sufficient heat energy.

In one embodiment, a heating element having the capacity to carry up to10 amperes, leading to watt densities at the ice interface of up to 30watt/in² for anti-icing operation is provided.

In one embodiment, the heating element is separate from the thermallyconducting, electrically insulating layer and the thermally conducting,electrically insulating layer lies between the heating element and thesurface skin to enable rapid conduction of heat to the ice/surfaceinterface.

In a second embodiment, the heating element consists of a combination ofelectrically conductive and thermally conductive materials. In thiscase, the heating element may be thermally conductive and optionallyelectrically conductive. Alternatively, the heating element may beelectrically conductive, and a second material present along with theheater may be thermally conductive, electrically conductive, or acombination of both. In this second embodiment, a thermally conducting,electrically insulating layer separates the heater from the surface skinand the ice interface.

In a third embodiment, a mixture of thermally and electricallyconductive nanoparticles is mixed with hexagonal boron nitride in such amanner that the conductive nanoparticles do not reach their percolationlimit, i.e. they do not touch each other and therefore form adiscontinuous network. However, the thermal conductivity of thenanoparticles provides a synergistic effect of enhancing the overallthermal conductivity of the mixture beyond that attainable without thepresence of the thermally and electrically conductive nanoparticles.

The film can be reinforced with carbon nanotubes (CNT), carbonnanofibers (CNF) or graphite nanoplatelets. The carbon nanotubes can bepresent in as little as from about 0.1 percent to about 5.0 percent byweight of the surface film. The CNT weight can be optimized to match theCTE of the hexagonal boron nitride film, to minimize microcracking andallow the use of a lighter surface film.

In another aspect of the invention, the film comprises metal coatedparticles, for example, silver-coated, hollow glass microspheres and/ormetal-coated carbon nanotubes (collectively, silver coated particles).While not wishing to be bound by a particular theory, it is believedthat the metal coating helps with the dispersion of the particles withinthe film. In one embodiment, a silver coating is applied onto carbonnanotubes by electroless plating, which is believed to improve theinterfacial adhesion of the composites to which the material is applied.The metal-coated particles can be subjected to pretreatments such asoxidation, sensitizing treatment and activation treatment, which canintroduce various functional groups on the particles. These functionalgroups can improve the dispersion of the particles into the film,increase the number of activated sites, and lower the deposition rate.

In another aspect, carbon nanotubes only can be added to the polymerfilm. The 10,10 armchair configuration carbon nanotube has a resistivityclose to copper and it is six times lighter than copper, and accordinglymay be a preferred nanotube. The nanotubes may be aligned throughvarious methods, including mechanical, chemical, and magnetic methods.For example, the nanotubes can be mixed with the polymerizable monomerand extruded into a film coating. The feed screw can be vibrated toimprove the alignment of fibers in the flow direction (similar tovibration injection molding used with recycled thermoplastics). Thenanotubes can be functionalized to react with the tail or head of eachnanofiber such that it will self-assemble (similar to lipid bi-layerassembly). This would require optimizing the nanotube loading so thatthe nanotubes attract each ether, while also ensuring that the epoxydoes not interfere with the process. Finally, the nanotubes can be madesuch that a nickel particle is attached to one end. Ferrous alloynanoparticles and carbon nanotubes (with the nickel particle) can beadded to the adhesive, primer, or paint and subjected to a magneticfield to align the nanotubes.

Metal nanorods/nanowires/nanostrands (collectively called nanowires) canalso be used. Carbon nanotubes or others nanoparticles that have beenmodified to decrease the resistivity can be added to the film. Thesemodified nanotubes can be oriented in-plane to replace or reduce themetal screen.

The use of carbon nanotubes, the metal powder/carbon nanotube blends,low density metal screens reinforced with carbon nanotubes, metal-coatedparticles and/or aligned graphite nanoplatelets can provide electricalconductivity to the thermally conductive film.

The compositions described above can be formed into thermally conductivefilms. The manner in which the films are formed depends, in part, on thenature of the compositions, i.e., whether they include thermoplasticmaterials or curable thermoset resins.

In one aspect, the films are formed by blending polymerizable monomersand hexagonal boron nitride, spreading the blend into a film thickness,and curing the monomers. In some embodiments, the blend is fairlyviscous at room temperature, for example, the consistency of paste ofputty, and can be warmed to temperatures above room temperature to thinthe blend to make it easier to spread.

In another aspect, the films are formed by blending thermoplasticpolymers and hexagonal boron nitride, and casting the blend into a film.In one aspect, the thermoplastic polymers are melted or softened beforeblending, or the blend is heated to an elevated temperature tofacilitate spreading.

The pre-cured composition comprising the hexagonal boron nitride, and,optionally, nanoparticulate conductive materials, can be placed on asurface of a composite structure via conventional means, such as knifecoating or doctor blading. In one embodiment, the hexagonal boronnitride and polymerizable monomer blend is spread into a thin layer andthe monomers are polymerized.

If the film includes too much hexagonal boron nitride, the film cansuffer from relatively low shear strength. Alternatively, if the filmdoes not include sufficient hexagonal boron nitride, there is notsufficient thermal conductivity. For this reason, the amount ofhexagonal boron nitride in the film is ideally in the range of fromabout 12 percent to about 40 percent by weight, based on the totalweight of the film. Preferably, the concentration of hexagonal boronnitride in the film is from about 27 percent to about 33 percent byweight, based on the total weight of the film. The hexagonal boronnitride provides the film with electrical conductance, whichadvantageously is between 10⁺¹⁰ and 10⁺¹⁵ ohm-cm.

The rate at which the film provides heating and/or cooling can beimportant. By uniformly mixing the hexagonal boron nitride in thepolymer material, there is a high degree of uniformity and a relativelyhigh heat-up rate. In addition to having a relatively high thermalconductivity, the material also has a relatively high “heat-up” rate. Inone embodiment the material has a thermal conductivity of 4.33 W/mK,which is very high. The heat-up rate, which is proportional to thermalconductivity, is rapid.

Although virtually any polymer can be used to form the films, there aresome situations where the films will be subjected to extreme heatvariations. For example, when used in aerospace applications,temperature variations from 120° F. to −140° F. might be observed. Thefilm must be capable of avoiding delamination under this type oftemperature variation. In these embodiments, epoxy resins are preferredfor forming such films. Representative epoxy resins include, but are notlimited to, diglycidyl ethers of bisphenol A(2,2-bis(4-hydroxyphenyl)propane)orsym-tris(4-hydroxkyyphenyl)propane,tris(4-hydroxyphenyl)methane, bisphenol F, tetrabromobisphenol A, theirpolyepoxide condensation products, cycloaliphatic epoxides,epoxy-modified novolacs (phenol formaldehyderesins) and the epoxidesderived from the reaction of epichlorohydrin with analine, o-, m- orp-aminophenol, and methylene dianaline

In one embodiment, the thermally conductive films of the presentinvention can form, in part, composite materials. The compositematerials described herein include various layers laminated to thethermally conductive film layer. These composite materials typicallyinclude superposed sheets, layers and plies. As illustrated in FIG. 1,the composite material 100 includes an insulating layer 110, anelectrically conductive layer 120, and an thermally conductive layer 130in one embodiment. Additional materials can be adhered over or underthese layers, and in some embodiments, the electrically conductive layer120 is also the thermally conductive layer 120. When used in aircraftapplications, the composite can be covered, for example, with one ormore metal layers used to form the “skin” of the aircraft, or,alternatively, can include a further layer of a composite material, suchas a carbon fiber layer.

As shown in the embodiment illustrated in FIG. 1, a composite layer withno electrical conductivity and low thermal conductivity (an insulatinglayer 110) typically lies beneath the electrically conductive layer 120.Insulating layers 110 provide insulation with respect to electricityand, also, ideally, with respect to heat.

Electrically conductive layers 120 provide the heat that the thermallyconductive layer 130 transfers to the outer surface. These layerstypically lie beneath, in some cases, directly beneath, the thermallyconductive layer 130, except in those embodiments where the thermallyconductive layer 130 is also an electrically conductive layer 120. Theelectrically conductive layer 120 can include components commonlypresent in typical electrothermal heaters and their constructions,including metal wires, foils, and mesh, which can be buried infiberglass or other thermally nonconducting materials.

The thermally conductive layer 130 has two faces, one of which is incontact with a source of heat, and another face which is or is incontact with a surface to be heated. The thermally conducting filmsdescribed herein can enhance the performance of these heaters, byincreasing the speed in which the heat is transferred. The compositematerials described herein can be used to prepare aircraft fuselage oran aircraft component.

Additional layers can include various prepregs, fabrics, honeycomb core,foam core, resin and adhesive layers. The structure may be fabricatedusing dry fabrics which are infused with resin using resin film infusionor resin transfer molding. The electrically conductively layer may alsobe laid up dry and infused with the polymer during compositefabrication. One or more of the layers in the composite material can bewoven materials including fiberglass, aramid, carbon prepreg, or otherfibers, and/or can include non-woven layers. In one embodiment, a layercapable of dissipating energy from a lightning strike (i.e., a lightningstrike protection layer) is placed over the thermally conductive layer.

The films, or composite materials including the films, can be used ascomponents of electrothermal heaters, which can be used for example, asan electro-thermal ice protection system for an airfoil. The films canbe used as part of a layered structure, and applied in virtually anylocation in an aircraft where heating is desirable. For example, thefilm can be one layer of a nacelle skin, or on a wing. The film can beused to provide heated floor panels.

Current resistive heaters formed from metal foils adhered to an adhesivefilm, with a pattern etched out to form ribbons, provide uneven heating.In contrast, electrothermal heaters formed from the films describedherein provide even heat. Indeed, by having the hexagonal boron nitridespread out evenly, the heat is also spread out evenly.

When the films are included in electrothermal heaters, the heaters caninclude an integral parting strip, with the film configurable to coverat least a portion of a leading edge of the airfoil with the integralparting strip disposed along an air-stagnation zone of the leading edge,and a controller coupled electrically to the film for controllingelectrical energy from a power source to the film in accordance with apulse duty-cycle and for controlling power to the parting strip of theheater to maintain the air-stagnation zone virtually free of iceformation.

The electrothermal heaters can be prepared by preparing the films, asdescribed above, in a rectangular shape having a length substantiallygreater than the width, suitable for covering at least a portion of aleading edge of the airfoil. Conductor wires can be attached to thefilm, or to a metal layer, such as a copper layer, adjacent to the filmlayer. The wires can be attached, for example, at each edge by using busbars.

Ideally, the heater includes a converter, powered by a suitable powersource, for supplying electrical heating energy to the heater oversource and return lines which are electrically isolated from the powersource. The converter ideally prevents the electrical heating energyfrom being conducted through the conductive structure of the airfoil. Anairplane can include a plurality of these electrothermal heaters, eachheater covering a segment of a leading edge of the airfoil, with acontroller coupled electrically to each of the heaters for multiplexingelectrical energy from a power source among the plurality of heaters inaccordance with a pulse duty-cycle. The plurality of heaters can includeparting strip areas comprising a multiplicity of differently shapedisland areas disposed on the heater surfaces within the parting striparea, where each island area can be separated from the other islandareas by the surface of the electrothermal heater.

In one embodiment, the electrothermal heater is coupled to a conductivestructure of the airfoil for distributing or dissipating electricalenergy of a lightning strike from the region through the conductor tothe conductive structure.

The electrothermal heaters and composite materials described herein canbe incorporated in or used to replace some or all of the compositematerials in aircraft components such as nacelles, fuselage, wings,stabilizers, and other surfaces in need of de-icing.

The heaters and materials can also be present in water heaters, airheaters, heated floor panels, electrothermal ice and erosion protection,potable water systems, hoses, pipes, ducting, walls, ceilings, heatedseats, heating pads, aid other articles that require protection fromfreezing or where the presence of heat would provide comfort and/orsafety. Such articles are well known to those skilled in the art.

Methods for manufacturing a composite material including the thermallyconducting films and/or electrothermal heaters described herein are alsodisclosed. In one aspect of the invention, the methods involve forming acomposite material without a thermally conductive layer, forming thethermally conductive films, and adhering the film to the remainder ofthe composite material, for example, using an adhesive layer or in-situcure.

The film thickness can range from 0.003 in to 0.010 in, and thethickness can be controlled using known methods for forming polymerfilms, such as calendaring, using a doctor blade, and the like. In thoseembodiments where a UV-polymerizable material is used, thepolymerization can be effected using ultraviolet light, and in otherembodiments, the polymerization reaction can be facilitated by exposingthe forming film to heat. The thermoset film can be polymerized in-situwith the thermoset composite layers or adhesively bonded secondarily. Athermoplastic film can be heated and pressed with thermoplasticcomposite layers, or bonded using resistance or ultrasonic welding andthe like. The film can also be adhesively bonded to thermoset orthermoplastic layers. The film can be laid up with dry fabric layers orprepreg for subsequent infusion of the polymer using resin transfermolding or resin infusion.

While the present invention has been described herein above inconnection with a plurality of aspects and embodiments, it is understoodthat these aspects and embodiments were presented by way of example withno intention of limiting the invention. Accordingly, the presentinvention should not limited to any specific embodiment or aspect, butrather construed in breadth and broad scope in accordance with therecitation of the claims appended hereto.

1. A film formed from a blend of hexagonal boron nitride and apolymerizable monomer, wherein the hexagonal boron nitride is present ina concentration of from about 12 percent to about 40 percent weight,based on total weight of the film.
 2. The film of claim 1, wherein thehexagonal boron nitride is present in a concentration of from about 27percent to about 33 percent weight, based on total weight of the film.3. The film of claim 1, wherein the monomer is a thermoset resinselected from the group consisting of epoxy based resin systems,matrices of bismaleimide (BMI), phenolic, polyester, PMR15 polyimide,acetylene terminated resins, acrylics; polyurethanes, and free-radicallyinduced thermosetting resins.
 4. The film of claim 1, wherein themonomer is an epoxy resin.
 5. The film of claim 4, wherein the epoxyresin is selected from diglycidyl ethers of bisphenol A(2,2-bis(4-hydroxyphenyl)propane) or sym-tris(4-hydroxyphenyl)propane,tris(4-hydroxyphenyl)methane, bisphenol, F, tetrabromobisphenol A, theirpolyepoxide condensation products, cycloaliphatic epoxides,epoxy-modified novolacs (phenoli-formaldehyde resins) and the epoxidesderived from the reaction of epichlorohydrin with analine, o-, m- orp-aminophenol, and methylene dianaline.
 6. The film of claim 1, furthercomprising an electrically conductive material.
 7. The film of claim 6,wherein the electrically conductive material is selected from the groupconsisting of metal powders, metal-coated microspheres, metal-coatedcarbon-nanotubes, carbon nanofibers, carbon nanotubes, graphitenanoplatelets, copper screen, and aluminum screen.
 8. The film of claim6, wherein the electrically conductive material is present in aconcentration of from about 10 percent to about 60 percent by weight ofthe hexagonal boron nitride.
 9. The film of claim 6, wherein theelectrically conductive material does not provide electricalconductivity.
 10. The film of claim 1, wherein the polymerizable monomerforms a thermoplastic polymer.
 11. The film of claim 10, wherein thethermoplastic polymer is selected from polyetheretherketone (PEEK),polyetherketone (PEK), polyphenylene sulfide (PPS), polyethylene sulfide(PES), polyetherimide (PEI), polyvinylidene fluoride (PVDF), polysulfone(PS), polycarbonate (PC), polyphenylene ether/oxide, nylons, aromaticthermoplastic polyesters, aromatic polysulfones, thermoplasticpolyimides, liquid crystal polymers, and thermoplastic elastomers. 12.The film of claim 10, wherein the hexagonal boron nitride is present inconcentrations of from about 5 percent to about 50 percent by weight ofthe thermoplastic polymer.
 13. A composite material comprising a filmand a thermally insulating layer, wherein the film is formed from ablend of hexagonal boron nitride and a polymerizable monomer, andwherein the hexagonal boron nitride is present in a concentration offrom about 12 percent to about 40 percent weight, based on total weightof the film.
 14. The composite material of claim 13, further comprisingan electrically conductive layer between the thermally insulating layerand the film.
 15. The composite material of claim 13, further comprisingan additional layer overlying the film, wherein the additional layer iscapable of dissipating energy from a lightning strike.
 16. A method forforming an thermally conductive film, comprising the steps of blendinghexagonal boron nitride and a polymerizable monomer wherein thehexagonal boron nitride is present in a concentration of between about12 and about 40 percent weight; forming a layer of the resulting blend;and polymerizing the polymerizable monomer to form the thermallyconductive film.
 17. A method for forming an thermally conductive film,comprising the steps of blending hexagonal boron nitride and a meltedthennoplastic polymer wherein the hexagonal boron nitride is present ina concentration of from about 12 to about 40 percent weight, based ontotal weight of the film; forming a layer of the resulting blend; andcooling the melted polymer to form the thermally conductive film.
 18. Amethod of electrothermally heating an airplane component, comprising thesteps of providing a thermally conductive film to the airplanecomponent, wherein the thermally conductive film comprises hexagonalboron nitride and a polymer; attaching at least one conductor to thethermally conductive film, wherein the conductor is further attached toa converter; and supplying electrical energy to the film via theconverter and conductor so as to heat the airplane component.
 19. Themethod of claim18, wherein the thermally conductive film is rectangularin shape with a length substantially greater than the width for coveringat least a portion of a leading edge of an airfoil.
 20. The method ofclaim 18, wherein the airplane component is selected from the groupconsisting of at least one nacelle, fuselage, wing and stabilizer.